Orthogonal Frequency Division Multiplexing (“OFDM”) is a multi-carrier data transmission technique that is advantageously used in radio-frequency based transmitter-receiver systems. These systems may include, for example, computer WiFi (IEEE 802.11a) data systems. Other standards where OFDM based system is used are IEEE 802.16 for a Fixed Wireless Access (FWA), HiperLAN2, Digital Audio Broadcast (DAB), Digital Video Broadcast (DVB) and Digital Subscriber Line (DSL).
OFDM systems typically divide available radio spectrum into many carriers. Each of the many carriers has a narrow bandwidth and is modulated with a low rate data stream. The carriers are closely spaced without causing inter-carrier interference (ICI) by ensuring that the carriers are orthogonal to each other.
When generating an OFDM signal, each carrier is assigned a data stream. The data streams are converted into symbols depending on the modulation scheme. For each symbol to be transmitted, phase and amplitude are calculated in the frequency domain. The phase and amplitude are determined according to the modulation scheme, which may be, for example, Quadrature Amplitude Modulation (QAM), Quadrature Phase Shift Key (QPSK), or any other suitable modulation scheme. Once the phase and amplitude are determined, they must be converted to time domain signals for transmission. Typically, OFDM systems use an Inverse Fast Fourier Transform (IFFT) to perform this conversion. The IFFT is an efficient way of mapping the data on to orthogonal carriers. The time domain signal is then up converted to the radio frequency of the appropriate carrier before transmission.
One of the problems associated with radio transmissions is inter-symbol interference (ISI) due to multi-path delay interference. A reflected radio signal follows a longer path than a line of sight radio signal. The difference in delay caused by reflected signals may obscure the direct signal. OFDM systems are well-suited to cope with multi-path delay problems because the low data rate of each carrier generates long symbol periods. Tolerance to multi-path delay is enhanced by the addition of Guard Intervals (GI) separating the symbols. If the Guard Intervals are as long or longer than the differential delays expected from multi-path sources, then multi-path interference is effectively eliminated. To achieve a reasonable throughput, the OFDM symbol duration may be at least five times the Guard Interval. In order to avoid inter-carrier interference (ICI), the OFDM symbol is cyclically extended in the GI.
Guard Interval insertion may be achieved as follows. An IFFT has an associated “length” corresponding to a number of coefficients for the transform. The Guard Interval is generated by outputting the last few IFFT output coefficients at the beginning of the symbol to form a cyclic prefix (CP). The size of the cyclic prefix varies for different applications. For example, for an IFFT with a length of 64, the output corresponding to the last 16 coefficients may be transmitted first as the cyclic prefix, and then the output corresponding to the 64 coefficients of the entire IFFT output in regular order.
For the addition of this CP in known systems, the IFFT output requires relatively large buffers. If the output of the IFFT is in bit-reversed time order, then two buffers of size N (N is length of IFFT) are required. If the output of the IFFT is in time order, then a single buffer of the length of the IFFT (N) is required. Buffers add expense to the system.
What is required is an IFFT and Guard Interval insertion stage that achieves the required signal processing with less buffer requirements.